1. Field of the Invention
This invention relates to an autotensioner for maintaining the tension of a belt constant, and more particularly to an autotensioner used to stably maintain the tension of a cam shaft driving timing belt, an auxiliary apparatus driving belt or the like of overhead cam (OHC) or dual overhead cam (DOHC) engines for automobiles.
2. Disclosure of the Related Art
The present invention is widely applicable where it is desired to stably maintain the tension of a belt constant, but the invention will be described with respect to the cam shaft driving timing belt of OHC or DOHC engines for automobiles as an example.
In an OHC or DOHC engine, the cam shaft for opening and closing the intake-exhaust values is located in the upper portion of the engine and lies at a location distant from the crank shaft rotatively driven by a piston(s). Accordingly, a cam shaft driving timing belt is used to transmit the rotation of the crank shaft to the cam shaft. This timing belt has teeth on the inner periphery thereof and is mounted between timing pulleys fixed onto the two shafts while meshing with the outer peripheral teeth of the pulleys and transmits the rotation.
It is necessary to impart a predetermined tension to the timing belt, and heretofore it has been the common practice to impart an initial tension to the timing belt by a fixed type tensioner. The fixed type tensioner is urged against the outer periphery of the timing belt by biasing means such as a spring and after a predetermined initial tension (e.g., 20-30 kg.f) is imparted to the timing belt, the fixed type tensioner is fixed onto the engine block by bolts or the like. This initial tension is imparted chiefly to prevent vibration of the timing belt and tooth skip between the timing belt and the timing pulley. However, the coefficient of linear expansion of the engine block formed of aluminum or like material is about four times as great as that of the timing belt of rubber having a wire embedded therein and therefore, the tension of the belt is greatly varied by a temperature change. That is, during low temperatures, the contraction of the distance between the pulleys is greater than the dimensional contraction of the belt and therefore, slack of the belt occurs and vibration of the belt increases and further, tooth skip may sometimes occur. Also, during high temperatures, the expansion of the distance between the pulleys is greater than the expansion of the belt and therefore, the tension of the belt increases to cause production of noise or reduction in the durability of the belt.
To solve the above-noted problems peculiar to the fixed type tensioner, there has been proposed a tensioner of the type which has the function of compensating for any variation in the tension of the belt (German Laid-Open Patent No. 2524744).
The tensioner of this type is generally called the autotensioner, and the autotensioner of said German Laid-Open Patent is comprised of a fixed portion fixedly held on the engine block, an annular eccentric wheel mounted at an eccentric location on the fixed portion and oscillating relative to and bearing against the belt, and a coil spring biasing the eccentric wheel toward the belt, and follows any fluctuation of the tension of the belt to maintain the tension.
The above-described autotensioner is of a construction in which the eccentric wheel is urged against the belt by the coil spring and therefore, when the load to the belt is great or when the load to the belt increases suddenly (for example, when the engine is started, suddenly stopped, suddenly braked or bites in foreign materials), a great tension or a suddenly increased tension is transmitted to the autotensioner as well and such tension overcomes the biasing force of the coil spring and the tensioner is also displaced together therewith. Moreover, where the rigidity of the coil spring is small, the displacement thereof is sensitive and great and therefore causes the belt to wave and further causes the problem of tooth skip. Particularly, during the starting at a low temperature, the problem of tooth skip is liable to occur.
Also, to provide a construction in which it is difficult for the tensioner to be displaced for a sudden increase in the tension of the belt, use may be made of a coil spring of high rigidity, but in this case, displacement is insensitive to the increase in belt tension occurring during high temperatures and the effect of alleviating the tension is small, thus giving rise to problems such as reduced durability of the belt and noise.
Accordingly, the conventional autotensioner cannot overcome the problem of imparting a moderate tension to the belt by a spring of small rigidity and yet absorbing a sudden increase in the tension of the belt to maintain a stable tension.
On the other hand, the tensioners disclosed, for example, in Japanese Laid-Open Patent Application No. 208343/1982 and Japanese Laid-Open Utility Model Application No. 49054/1984 are of a construction in which an elastic shock absorbing member is disposed in the radial gap between a rotating portion rotatable while bearing against the belt and a fixed portion attached to the engine block, and the shock absorbing member is compressively deformed in accordance with an increase in the tension of the belt to thereby reduce the increase in the tension of the belt.
In the conventional tensioner of the above-described construction, the shock absorbing member is always subjected to the belt tension load and is therefore plastically deformed with the lapse of time. By this plastic deformation of the shock absorbing member, the tension of the belt is reduced and thus, the initial tension cannot be maintained, and this has led to the possibility that problems such as increased vibration of the belt and tooth skip may occur.